Chemical vapor deposition apparatus and method

ABSTRACT

Chemical vapor deposition apparatus and method are provided with coating gas distribution and exhaust systems that provide more uniform coating gas temperature and coating gas flow distribution among a plurality of distinct coating zones disposed along the length of a coating chamber.

This is a continuation of Ser. No 09/950,013 filed Sep. 10, 2001, nowU.S. Pat. No. 6,793,966.

FIELD OF THE INVENTION

The present invention relates to a chemical vapor deposition (CVD)apparatus and method for applying coatings to substrates.

BACKGROUND OF THE INVENTION

Chemical vapor deposition (CVD) involves the generation of metal halidegas at low temperatures (e.g. about 100 to 600 degrees C.), introductionof the metal halide gas into a high temperature retort (e.g. 200 to 1200degrees C. retort temperature), and reaction of the metal halide withsubstrates positioned in the retort to form a coating thereon. Ingeneral, a large excess of metal halide gas is used to prevent reactantstarvation in the high temperature coating retort. CVD processestypically are conducted at reduced pressure (subambient pressure). CVDapparatus and method are described in Howmet U.S. Pat. Nos. 5,261,963and 5,263,530. Howmet U.S. Pat. No. 6,143,361 described CVD apparatusand method wherein deposition of excess metal halide reactant in thecoating gas exhausted from the coating retort is reduced or eliminatedto reduce retort downtime required to remove deposits from the retortexhaust system.

The CVD process can be used to codeposit Al, Si, and one or morereactive elements such as Hf, Zr, Y, Ce, La, etc. to form protectivealuminide diffusion coatings on substrates such as nickel and cobaltbase superalloys commonly used to cast gas turbine engine airfoils.Copending U.S. Ser. Nos. 08/197,497 and 08/197,478 disclose CVDapparatus and method to produce protective reactive element-modifiedaluminide diffusion coatings. U.S. Pat. No. 5,989,733 describes aprotective outwardly grown, platinum-modified aluminide diffusioncoating containing Si and Hf and optionally Zr, Y, Ce, and/or La formedon a nickel or cobalt base superalloy substrate by such CVD apparatusand process.

There is a need to provide improved CVD apparatus and method that arecapable of producing aluminide diffusion coatings modified by inclusionof one or more other coating elements, such as for example only siliconand one or more so-called reactive elements, wherein the coatings can beproduced having a more uniform coating composition, microstructure, andthickness throughout the working volume (throughout multiple coatingzones) of the CVD coating apparatus. It is an object of the presentinvention to satisfy this need.

SUMMARY OF THE INVENTION

In one embodiment of the present invention, CVD apparatus and method areprovided with an improved coating gas distribution system to providemore uniform coating gas temperature among a plurality of coating zonesin a coating chamber.

In another embodiment of the present invention, CVD apparatus and methodare provided with an improved coating gas distribution system to providemore uniform flow of coating gas among a plurality of coating zones inthe coating chamber.

In still another embodiment of the present invention, CVD apparatus andmethod are provided with an improved coating gas exhaust system thatreduces interaction between the inlet coating gas flow to each coatingzone and exhaust gas flow from each coating zone so as provide a moreuniform gas flow pattern in each coating zone.

The above and other objects and advantages of the present invention willbecome more readily apparent from the following detailed descriptiontaken with the following drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a somewhat schematic view of a CVD coating gas generator and acoating reactor chamber that is shown in a longitudinal sectional viewpursuant to an embodiment of the invention.

FIG. 2 is an enlarged longitudinal sectional view of the coating reactorchamber and coating gas distribution system pursuant to an embodiment ofthe invention.

FIG. 3 is an enlarged longitudinal sectional view of the externalcoating gas generator.

DESCRIPTION OF THE INVENTION

For purposes of illustration and not limitation, the present inventionwill be described herebelow with respect to a CVD apparatus and methodfor producing a protective platinum-modified aluminide diffusion coatingcontaining Si, Hf and optionally Zr on a nickel base superalloysubstrate of the type disclosed in U.S. Pat. No. 5,989,733, theteachings of which are incorporated herein by reference. Zr can bepresent in the coating as a result of being an impurity in the Hfpellets described below or as an intentional coating addition. Theinvention is not limited to making such coatings and can be practiced toform other coatings on other substrates.

Referring to FIGS. 1-2, CVD coating apparatus pursuant to an embodimentof the invention comprises a reactor or retort 12 adapted to be disposedin a refractory-lined heating furnace 14 shown schematically that isused to heat the retort 12 to an elevated CVD coating temperature. Thefurnace 14 can be an electrical resistance or other known type offurnace to this end. Metallic substrates SB to be coated are placed in acoating reactor chamber 20 disposed in the retort 12 and are heated byradiation from the walls of the heated retort.

The retort 12 includes a lid 16 to close off the upper end of theretort. To this end, the retort lid 16 is air-tight sealed on a flange12 f of the retort by an O-ring seal 17. The flange 12f includes anannular water cooling passage 12 p through which passage water iscirculated to cool the flange during operation of the retort. Lid 16includes an annular chamber 16 a receiving a thermal insulation block ormember 16 b therein to reduce heat losses from the retort. Components ofthe coating reactor chamber 20 can be supported on the lid 16 and thenlowered with lid 16 into the retort 12. The coating reactor chamber 20includes conduits 18, 22 joined at connection 57, which connection ismade prior to closing the lid 16 on the retort 12. Conduit 22 is part ofthe retort cover 16 as a result of being welded thereto.

The retort lid 16 includes central coating gas inlet conduit 22 throughwhich reactive coating gases are supplied to the axial gas preheat anddistribution conduit 18 of the reactor 20 as described below. Theconduit 18 includes an inner axial gas preheat conduit 52 therein. Thecoating reactor chamber 20 comprises a plurality of distinct annularcoating zones 24 a, 24 b, 24 c (FIG. 2) at different axial elevations inthe retort and disposed about coating gas preheat and distribution pipeor conduit 18. Referring to FIG. 2, substrates SB to be coated aredisposed on trays 28 in the coating zones 24 a, 24 b, 24 c. The trays 28close off the coating zones 24 a, 24 b, 24 c. The coating zones areshown disposed one atop another for purposes of illustration and notlimitation since fewer or greater number of coating zones can beemployed in practice of the invention.

Referring to FIG. 1, the coating gas inlet conduit 22 is communicated toa plurality of low temperature metal halide generators 30 of identicalconstruction with the exception of the metal charge B therein, FIG. 3.The metal charge B in each generator 30 is different and selected togenerate a particular coating gas constituent, such as for purposes ofillustration and not limitation, an aluminum or aluminum alloy pelletbed in generator #1 to generate aluminum trichloride or other volatilealuminum halide coating gas constituent, a silicon or silicon alloypellet bed in generator #2 to generate silicon tetrachloride or othervolatile silicon halide coating gas constituent, and a reactive element,such as Hf or an alloy thereof, in generator #3 to generate a hafniumtetrachloride or other volatile hafnium halide coating gas constituent.Other reactive elements that can be used in lieu of, or in addition toHf or its alloys, include Zr and its alloys, Ce and its alloys, andNi—Mg alloys to generate a Mg-bearing coating gas.

The generators 30 are located externally of the retort 12 and connectedto inlet conduit 22 via heated conduits 32. The conduits 32 are heatedby conventional heating devices, such as electrical resistance heatedflexible tapes or electrical resistance heated rods or sticks, toprevent condensation of the metal halide coating gases therein.

For producing a protective platinum-modified aluminide diffusion coatingcontaining Si, Hf and Zr on a nickel base superalloy substrate of thetype disclosed in U.S. Pat. No. 5,989,733, the first metal halidegenerator #1 is used to generate aluminum trichloride or other aluminumhalide coating gas constituent. The generator is supplied with a gasflow F1 comprising a mixture of an acid halide gas, such as typicallyHCl or other hydrogen halide gas, and a reducing or inert carrier gas,such as hydrogen, argon, helium, or mixtures thereof, via conduits 33from suitable sources 41, 42, such as respective high pressure cylindersor bulk cryogenic supplies. The acid halide gas and carrier gas areblended together in suitable proportions to provide the gas flow F1 tothe first generator.

Referring to FIG. 3, the first generator #1 includes a bed B of aluminummetal pellets and an heating device 46, such as an electrical resistanceheater, to heat the Al pellets to a reaction temperature depending uponthe acid halide gas supplied to the generator. For example only, analuminum pellet temperature of about 200 degrees C. or higher can beused for HCl gas. The pellet temperature for other hydrogen halide gasesdepends on the boiling point of the aluminum halide formed in thegenerator. The acid halide gas/carrier gas flow F1 is supplied togenerator #1 to flow over the Al pellets under conditions oftemperature, pressure, and flow rate to form aluminum trichloride orother aluminum halide gas, depending on the hydrogen halide gas used, inthe carrier gas. Typical temperature, pressure, and flow rate to formaluminum trichloride at generator #1 are as taught in U.S. Pat. No.5,658,614 as follows:

-   -   Hydrogen halide/carrier gas—13 vol. % HCl; balance H₂    -   Pellet temperature—290 degrees C.    -   Flow rate—46 scfh (standard cubic feet per hour)

The second metal halide generator #2 is used to generate silicontetrachloride or other volatile silicon halide coating gas constituent.The generator is supplied with a gas flow F2 comprising a mixture of anhydrogen halide gas, such as typically HCl gas, and a reducing or inertcarrier gas, such as hydrogen, helium and argon, or mixtures thereof,from suitable sources 41, 42, such as respective high pressure cylindersor bulk cryogenic supplies. The hydrogen halide gas and carrier gas areblended together in suitable proportions to provide the gas flow F2 tothe second generator. The second generator #2 includes a bed B ofsilicon pellets and heating device 46, such as an electrical resistanceheater, to heat the Si pellets to a reaction temperature depending uponthe acid halide gas supplied to the generator. For example only, asilicon pellet temperature of about 100 degrees C. or higher can be usedfor HCl gas. Pellet temperatures for other hydrogen halide gases dependson the boiling points of the silicon halide formed in the generator.Typical temperature, pressure, and flow rate to form silicontetrachloride at generator #2 are as follows:

-   -   Hydrogen halide/carrier gas—2 vol. % HCl; balance H₂    -   Pellet temperature—290 degrees C.    -   Flow rate—26 scfh

The third metal halide generator #3 is used to generate a reactiveelement chloride or other reactive element halide gas, such as hafniumtetrachloride coating gas constituent. The generator is supplied with agas flow F3 comprising a mixture of an acid halide gas, such astypically HCl gas, and an inert carrier gas, such as argon, helium, ormixtures thereof, from suitable sources 43, 44, such as respective highpressure cylinders or bulk cryogenic supplies. The hydrogen halide gasand carrier gas are blended together in suitable proportions to providethe gas flow F3 to the first generator. The third generator #3 includesa bed B of hafnium pellets containing natural Zr impurities and heatingdevice 46, such as an electrical resistance heater, to heat the Hfpellets to a reaction temperature depending upon the acid halide gassupplied to the generator. For example only, a hafnium pellettemperature of about 430 degrees C. can be used for HCl gas. Pellettemperatures for other hydrogen halide gases depends on the boiling orsublimation points of the metal halide formed in the generator. Thepellets of the bed in generator #3 can comprise an alloy of Hf and Zr inthe event Zr is to be intentionally present as an alloyant in thecoating. Typical temperature, pressure, and flow rate to form hafniumtetrachloride at generator #3 are as follows:

-   -   Acid halide/carrier gas—3 vol. % HCl; balance Ar    -   Pellet temperature—430 degrees C.    -   Flow rate—33 scfh

In lieu of having three separate generators, a cogenerator can be usedto cogenerate two metal halide gases. For example, aluminum trichlorideand silicon tetrachloride can be cogenerated by flowing a hydrogenhalide/carrier gas mixture first over a bed of Al pellets and then overa bed of Si pellets located downstream of the bed of aluminum pellets asdescribed in copending application Ser. No. 08/197 478, the teachings ofwhich are incorporated herein by reference, to generate a coating gasconstituent that includes both AlCl₃ and SiCl₄ in proportions controlledby the flow rate of the acid halide/carrier gas over the beds. The thirdgenerator #3 would still be used to generate the HfCl₄ coating gasconstituent. Alternately, hafnium tetrachloride and silicontetrachloride can be cogenerated by flowing a hydrogen halide/carriergas mixture first over a bed of Hf pellets and then over a bed of Sipellets located downstream of the bed of hafnium pellets. Anycombination of pellet beds where metal halide gas from the firstupstream bed is more stable than a second metal halide formed in thesecond downstream bed can be used as a cogenerator in practice of theinvention

The coating gas constituents from generators 30 are supplied to theinlet conduit 22 connected to the gas preheat and distribution conduit18 at connection 57. A suitable pump P, such as vacuum pump, isconnected to the exhaust 80 of the reactor coating chamber in a mannerto maintain a desired pressure, desired flow rate of the gases throughthe generators 30 and the coating chamber 20 and to exhaust spentcoating gas from the coating chamber.

The metal halide generators 30 are constructed to reduce leakage of airinto the generators at the inlet fitting 30 a, outlet fitting 30 b andflange joint 30 c thereof. Each generator 30 is identical to the otherexcept for the bed B of pellets therein.

In FIG. 3, generator 30 is shown including a metal (e.g. stainlesssteel) housing 30 h having electrical resistance heating device 46disposed thereabout to heat the bed B in the generator to a desiredreaction temperature; for example, as described above. The housing 30 hincludes an annular, laterally extending flange region 30 f at a lowerend to rest on a generator base 35 with an O-ring seal 33 therebetween.The flange region 30 f resting on base 35 defines joint 30 c. The flange30 f includes an annular passage 30 p through which cooling fluid (e.g.water) is flowed during operation of the generator to cool the flangeand maintain its temperature in the range of about 40 to about 100degrees C. for purposes of illustration and not limitation. Cooling offlange region 30 f during operation of the generator 30 reducesdistortion of the flange region 30 f from the elevated temperature ofthe housing 30 h during generator operation, and minimizes oxidation ofthe O-ring.

The O-ring seal 33 is compressed between the cooled flange region 30 fand flange 35 f of the generator base 35 to provide an air-tight sealtherebetween. The O-ring seal comprises an acid resistantfluoroelastomer polymeric material that does not release carbon, sulfuror other unwanted tramp element into the generator 30 that couldadversely affect the coating produced on substrates SB. A suitableO-ring 33 is commercially available as a Viton O-ring from Dupont DowElastomers, Wilmington, Del. More than one O-ring seal 33 can beprovided between flange region 30 f and base 35.

The inlet fitting 30 a on base 35 and outlet fitting 30 b on housing 30h of the generator 30 comprise commercially available zero clearancefittings that provide knife-edge sealing surfaces (not shown) thatpenetrate into an annular nickel gasket (not shown) to provide anair-tight seal. Suitable zero clearance fittings 30 a, 30 b areavailable as VCR metal gasket and face seal fittings from SwagelokCorporation, Solon, Ohio.

The bed B of pellets is disposed on a perforated gas distribution plate37 that is positioned further downstream of the flange region 30 f; i.e.downstream in the direction of flow of the gases in the generator, so asto reduce heat input to the flange region 30 f and O-ring seal 33. Inthe past as disclosed in U.S. Pat. Nos. 5,407,704 and 5,264,245, theplate 37 was positioned at the flange region 30 f having a grafoilgasket that emitted carbon and sulfur into the generator. The plate 37is heated by contact with the bed B of pellets in the generator and byproximity to the heater 46 such that the more remote positioning of theplate 37 from the flange region 30 f reduces heat input to the flangeregion 30 f and O-ring seal 33. A typical spacing of the gasdistribution plate 37 from the flange region 30 f is 1 inch or more forpurposes of illustration and not limitation.

Reduction of air leaks into the generators 30 at the flange joint 30 fand fitting 30 a reduces oxidation of the pellet charge forming bed B.Thus, the efficiency of utilization of the pellet charges is increased.For example, the efficiency of use of the hafnium pellet charge ingenerator #3 was improved from less than 5% to more than 98% byprevention of air leaks into the third generator 30. Reduction of airleaks into the coating gas conduit at fitting 30 b prevents oxidation ofthe reactive element halides exiting the generator and so improvescontrol of the coating composition.

Pursuant to an embodiment of the present invention, an improved coatinggas distribution system is provided to provide more uniform coating gastemperature among the coating zones 24 a, 24 b, 24 c in the coatingchamber 20. In particular, the coating gas constituents (e.g. AlCl₃,SiCl₄, HfCl₄ and carrier gases) are conveyed to inlet conduit 22 whichdefines a gas manifold 50 that is located above and upstream of thecoating chamber 20 in the retort 12 and that communicates withupstanding inner coating gas preheat conduit 52 inside coating gaspreheat and distribution conduit 18 such that the coating gas stream ST(comprising the coating gas constituents) entering the inlet conduit 22flows through the manifold 50 and down the preheat conduit 52 to thelowermost coating zone 24 c of the coating chamber 20 and back up in theannular space between the conduits 18 and 52 in a manner that thecoating gas stream ST is preheated before entering the coating zones 24a, 24 b, 24 c via conduit 18. The manifold 50 includes a heater device54, such an elongated electrical resistance heater, suspended thereinsuch that gas stream ST flows about the heater device 54 to heat the gasstream ST. A suitable electrical resistance heater that can be placed inmanifold 50 is commercially available as Firerod Cartridge from WatlowCorporation, St. Louis, Mo., although other heating devices can be usedto this end. The heater device 54 can be suspended along the length ofthe manifold 50 by a conventional swaglock compression connection 55.

The inlet conduit 22 communicates to preheat conduit 52 that residesinside coating gas preheat and distribution conduit or pipe 18. Theconduit 22 and conduits 18, 52 are connected by a union type pipefitting connection 57.

The conduit 52 extends axially through and along the length of theretort 12 through the coating zones 24 a, 24 b, 24 c disposed along thelength of the coating chamber 20 to the lowermost coating zone 24 cwhere the conduit 52 includes a lower gas discharge opening 52 a todischarge the coating gas stream ST into the annular space between thegas preheat and distribution conduit or pipe 18 and preheat conduit 52for flow upwardly in the annular space to the coating zones asillustrated by the arrows.

For purposes of illustration and not limitation, the exemplary coatinggas stream ST described above (e.g. AlCl₃, SiCl₄, HfCl₄ and carriergases) can be preheated to a gas temperature of greater than 100 degreesC. by the heater device 54 in the manifold 50 and the heating providedby flowing the stream through conduits 18, 52 in the above describedmanner when the coating chamber 20 is at a temperature of 1080 degreesC.

In addition, radiant heat shields 70 are provided above the coatingzones 24 a, 24 b, 24 c to reduce heat losses from the top of the coatingchamber 20. The heat shields 70 comprise stainless steel plates fastenedin the parallel arrangement illustrated above the coating chamber 20 toreflect radiant heat energy back toward the coating chamber 20. The heatshields 70 include legs 70 a spaced circumferentially about theirperipheries so that the plates 70 can stacked atop one another on theupper tray 28. Such radiant heat shields 70 can be used in lieu of thegettering screens described in U.S. Pat. No. 5,407,704.

Preheating of the coating steam ST using the heater device 54 in themanifold 50 and using the heating provided by flowing the stream throughconduits 18, 52 in the above described manner as well as reduction ofradiant heat losses from the coating chamber 20 by shields 70 improvesuniformity of the coating gas temperature in the coating zones 24 a, 24b, 24 c to dramatically reduce coating thickness variations onsubstrates SB from one coating zone to the next. That is, the coatinggas stream ST is more uniformly heated in the retort 12 to the desiredcoating deposition temperature prior to its being directed into thecoating zones 24 a, 24 b, 24 c by practice of this embodiment of theinvention. For purposes of illustration and not limitation, a coatinggas stream temperature gradient of only 50 degrees F. over the length ofthe coating chamber 20 can be provided as compared to a 400 degrees F.temperature gradient experienced in CVD apparatus of the typeillustrated in U.S. Pat. Nos. 5,407,704 and 5,264,245.

Once the coating gas stream ST has reached a desired reaction or coatingtemperature, another embodiment of the invention provides an improvedcoating distribution system to provide more uniform distribution of thepreheated coating gas stream among the coating zones 24 a, 24 b, 24 c inthe coating chamber 20.

In particular, the preheat and distribution conduit 18 extends axiallythrough the annular substrate support trays 28 which define therebetweenthe distinct annular coating zones 24 a, 24 b, 24 c about pipe orconduit 18. The pipe or conduit 18 includes at a mid-point of the heightof each coating zone 24 a, 24 b, 24 c a plurality of circumferentiallyspaced apart gas discharge holes or openings 62 to discharge thepreheated coating gas stream ST to each coating zone. The number ofopenings 62 at each coating zone can be varied as desired. For adiameter of conduit 18 of 1½ inches and axial spacing of 6 inchesbetween trays 28, three or more openings 62 can be provided in conduit18. The area of the openings 62 (e.g. number of holes) at the coatingzones 24 a, 24 b, 24 c is systemically varied to provide equal coatinggas flow from conduit 18 to each coating zone. Typically, the number ofopenings 62 at coating zone 24 a are greater than those at coating zone24 b, and the number of holes 62 at coating zone 24 b is greater thanthose at coating zone 24 c. For example only, the number of holes atcoating zone 24 a can be 10, the number of holes at coating zone 24 bcan be 8, and the number of holes at coating zone 24 c can be 6.

The conduit 52 also includes one or more bleed openings 52 b above thelower primary coating gas discharge opening 52 a for discharging coatinggas along the length of conduit 52. For example, one bleed opening 52 bis located at coating zone 24 b and one bleed opening 52 b is located atcoating zone 24 c to assist in providing generally equal flow of coatinggas among the coating zones 24 a, 24 b, 24 c. Although one bleed opening52 b is shown at each coating zone 24 b and 24 c in the upper region ofeach coating zone 24 b, 24 c to this end, more than one bleed openingcan be provided at the same or different locations at coating zones 24a, 24 b, 24 c as needed to generally equalize the flow of coating gasamong the coating zones 24 a, 24 b, 24 c. The coating gas dischargedfrom bleed openings 52 b flows upwardly in conduit 18 to this end. Bleedopenings 52 b each having a diameter of 0.125 inch can be provided tothis end for use with conduits 18, 52 having dimensions describedherein.

The annular trays 28 are spaced axially apart proximate their innercircumference by upstanding spacer annular inner walls 64 and proximatetheir outer circumference by upstanding outer perforated baffles 66. Thespacer walls 64 are positioned symmetrically about pipe or conduit 18 byretaining rings 67 welded or otherwise provided on trays 28. The trays28 include a central hole 28 a having inner diameter about equal to theouter diameter of pipe or conduit 18 to receive same in manner that thetrays 28 are symmetrically disposed about the pipe or conduit 26. Thetrays 28, spacer walls 64, and baffles 66 are stacked atop one anotherand supported on a lowermost, laterally flange 18 a of the pipe orconduit 18. The gas distribution pipe or conduit 18, trays 28, spacerwalls 64 and baffles 66 thereby are arranged in fixed positionssymmetrically about the central longitudinal axis of the coating chamber20.

The spacer walls 64 form an annular gas manifold 68 at each coating zone24 a, 24 b, 24 c between the pipe or conduit 18 and walls 64 each ofwhich provides a manifold wall. Each spacer wall 64 opposes or faces thegas discharge openings 62 of the pipe or conduit 18 at that coatingzone. Each spacer wall 64 includes first and second sets ofcircumferentially spaced apart gas flow opening openings 65 located anequal distance above and below the height of the openings 62 in pipe orconduit 18. Each spacer wall 64 thereby is provided with a plurality ofgas flow openings 65 that are out of alignment with the gas dischargeopenings 62 at each coating zone such that there is no line-of-sight gasflow path from the gas discharge openings 62 to gas flow openings 65 ateach coating zone.

For purposes of illustration and not limitation, 48 gas flow openings 65having a diameter of 0.25 inches can be provided in each wall 64 at eachcoating zone 24 a, 24 b, 24 c when the conduit 18 includes opening 62whose number and diameters are described above. Locating the openings 62of the gas distribution pipe or conduit 18 midway between the sets ofopenings 65 prevents gas jets from flowing directly across the eachcoating zone. Also, deflection of the coating gas off of the inside ofthe wall 64 at each coating zone produces more uniform gas flow aboutthe circumference of each coating zone 24 a, 24 b, 24 c.

The above gas distribution system provides a uniform and repeatable gasflow to the coating zones 24 a, 24 b, 24 c to improve coatingcomposition and microstructure uniformity among substrates SB on thesame tray 28 and among substrates in different coating zones.

Once the coating gas stream ST has flowed over the substrates SB ontrays 28 at each coating zone 24 a, 24 b, 24 c, still another embodimentof the present invention provides an improved spent gas exhaust systemto provide less interaction between the inlet coating gas flow to eachcoating zone 24 a, 24 b, 24 c and the exhaust gas flow from each coatingzone so as provide a more uniform flow pattern of coating gas in thecoating zones.

In particular, perforated tubular baffles 66 are provided between thetrays 28 at their outer circumferences as shown in FIG. 1-2. The tubularbaffles 66 comprise IN-600 nickel base superalloy and include patternsof exhaust openings 66 a through which spent (exhaust) gas from thecoating zones 24 a, 24 b, 24 c is exhausted. The pattern of openings 66a as well as their number and size (e.g. diameter) can be selected toprovide more or less uniform gas flow pattern at each coating zone 24 a,24 b, 24 c. For purposes of illustration and not limitation, a suitablepattern of openings 66 a is shown in FIG. 1 wherein each baffle 66includes 90 openings 66 a with each opening having a diameter of 0.375inch. Such baffles 66 can be used with the diameters and numbers ofopenings 62 on pipe or conduit 18 and openings 65 on spacer walls 64described above to provide a more uniform gas flow pattern from theinner to the outer circumference of each coating zone 24 a, 24 b, 24 cto in turn improve uniformity in the composition and microstructure ofthe diffusion aluminide coating (or other coating) formed on thesubstrates SB.

The spent gas exhausted through baffle openings 66 a flows to an exhausttube or conduit 80 that communicates to exhaust gas treatment equipmentas described in U.S. Pat. No. 6,143,361, the teachings of which areincorporated herein by reference. The countercurrent flow of exhaust gasoutside of inlet conduit 22 helps preheat the coating gas flowingtherethrough via heat exchange between the exhaust gas and coating gasin conduit 22. Although the invention has been described with respect tocertain embodiments, those skilled in the art will appreciate that theinvention is not so limited to these embodiments since changes,modifications, and the like can be made thereto within the scope of theinvention as set forth in the appended claims.

1. A method of chemical vapor deposition, comprising flowing coating gasin a gas distribution conduit in a coating chamber, discharging thecoating gas laterally from the gas distribution conduit to an annularmanifold located about the gas distribution conduit laterally betweenthe gas distribution conduit and an annular coating zone and having gasflow openings of the manifold communicated to the coating zone, andexhausting spent coating gas from the coating zone through openings inan annular baffle located about and outwardly of the coating zone.
 2. Amethod of chemical vapor deposition, comprising flowing coating gas in agas distribution conduit in a heated coating chamber, discharging thecoating gas from the gas distribution conduit laterally through gasdischarge openings at an opposing manifold wall of an empty manifolddisposed laterally between a coating zone and said gas distributionconduit, and flowing the coating gas through a plurality of gas flowopenings disposed in the manifold wall to the coating zone with the gasflow openings being out of alignment with said gas discharge openingssuch that there is no line-of-sight gas flow path from said gasdischarge openings to said gas flow openings through said empty manifoldto the coating zone.